Solar sails are all about "propellantless" propulsion — using photons from the Sun (as opposed to an onboard fuel supply) to push a spacecraft through space. Building on the lessons learned from smaller solar sail missions, like NASA's NanoSail-D and JAXA's IKAROS, NASA is gearing up to to launch the biggest solar… »2/01/13 10:40am 2/01/13 10:40am